The present invention relates to improvement in speed control systems for automobiles. More particularly, the invention relates to speed control system to maintain the speed of an automobile at stabilized desired by value by employing a negative feedback loop.
Speed controls for automobiles are now in commercial use, in which actual speed of the automobile is compared with a preselected desired value to generate a throttle position feedback signal for controlling the throttle valve at a position at which the actual speed balances with the preselected value. Generally speaking, automatic speed control of automobiles requires stabilization of the speed control system. If the stabilization should be not sufficient, so called hunting, overshoot or undershoot of the actual speed of the automobile occurs. The hunting can be prevented by deteriorating the responsibility and accuracy of the speed control system. However, the hunting can also be prevented without deteriorating this responsibility by employing negative feedback control of the throttle valve as shown in U.S. Pat. No. 3,381,771. (issued May 7, 1968). In the invention of the patent, actual speed indication signal from a tachometer generator of an automobile speedometer and a preselected desired speed indication signal from a speed set potentiometer are supplied to a comparator circuit to generate an error signal to control the throttle valve position, then the error signal is supplied to a differential amplifier circuit together with a throttle position feedback signal (the negative feedback signal) from a feedback potentiometer linked with the throttle valve. The output signal of the differential amplifier circuit is supplied to a vacuum modulator which in response thereto provides to a vacuum motor vacuum pressure related to the output signal. The vacuum motor is mechanically ganged to the throttle valve and controls its position. Thus the output differential signal of the differential amplifier regulates the position of the throttle valve. The negative feedback of the throttle valve position prevents extra movement of the throttle valve toward opening or closing. Therefore over acceleration and deceleration of the automobile are prevented. However, this negative feedback of throttle valve position is relatively difficult to implement because a transducer such as the feedback potentiometer which is linked with a throttle drive system (from an accelerator pedal to the throttle valve) and which generates a throttle position feedback signal is required, and the space to accommodate the transducer is limited (since the throttle drive system is installed in a narrow space). Moreover, electrical lead installation from the transducer to the differential amplifier circuit as well as mechanical adjustment of the linkage between the throttle drive system and the transducer are required. The feedback potentiometer may be weared out too early due to mechanical reciprocal operation or vibration, temperature, dampness and/or dust in engine room of the automobile.
Similar speed controls for automobiles are disclosed in U.S. Pat. Nos. Re. 27,324 (issued Mar. 28, 1972), 3,477,346 (issued Nov. 11, 1969), 3,485,316 (issued Dec. 23, 1969), 4,056,157 (issued Nov. 1, 1977). Negative feedback circuit without use of the feedback potentiometer, and which compares a speed error signal with an actual automobile speed signal to generate a negative feedback signal, is disclosed in U.S. Pat. No. 3,952,829 (issued Apr, 27, 1976). Also negative feedback circuit without use of the feedback potentiometer, which has a circuit means for retarding speed error signal to be supplied to desired speed signal generator circuit as the negative feedback signal is disclosed in U.S. Pat. No. 3,793,622 (issued Feb. 19, 1974).